Method and apparatus for wrapping a load using variable roller stretch

ABSTRACT

A method and apparatus for wrapping the load at a desired wrap force are provided. The apparatus includes a dispenser for dispensing packaging material and means for providing relative rotation between the load and the dispenser. The dispenser has a prestretch portion and a post-stretch portion. The prestretch portion is configured to prestretch the packaging material within the dispenser to a point below the yield point of the packaging material. Preferably the packaging material is prestretched between first and second unpowered prestretch rollers. The post-stretch portion is configured to further stretch the prestretched packaging material to a point above the yield point of the packaging material as the packaging material moves from the dispenser to the load. Preferably, the post-stretch portion includes a friction element, such as a brake, to apply friction to one of the prestretch rollers.

FIELD OF THE INVENTION

[0001] The invention relates to wrapping a load with packaging material, and, more particularly, to stretch wrapping.

BACKGROUND OF THE INVENTION

[0002] Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique which dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by a turntable, rotating arm, or horizontal rotating ring, typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.

[0003] The elasticity of the stretched plastic film compresses the products of the load to hold them together. The effectiveness of stretched packaging material in holding a load together is a function primarily of the cumulative containment force being placed on the load and secondarily the ultimate strength of the total layered packaging material wrap. These two functions are determined by the capability of the film to conform to the shape of the load at the optimum wrap force and the ultimate strength of the packaging material after it has been applied to the load.

[0004] Most stretch wrap packaging materials, when stretched above their yield point, gain significantly in modulus or stiffness. They ultimately become more cellophane like and less rubbery. The typical packaging material will multiply three times the ultimate strength per square inch of cross-sectional area after being elongated approximately 300 percent. The significant increase in modulus begins approximately when the yield point of the packaging material is exceeded during elongation or stretching. The yield point for packaging materials has increased substantially in the last twenty years. The yield point is achieved between 60% and 100% stretch for most currently available stretch wrapping materials.

[0005] Therefore, while packaging materials gain in strength as they are elongated past their yield points, they also experience an increase in their hardness, i.e., they become more brittle. This is of particular importance when wrapping “order picked” or non-standard type loads, where the shape of the load is irregular. Such irregularly shaped loads often have corners or points of contact which place a high stress on the stretched, brittle film, causing the film to tear or puncture. Thus, while it is important that the packaging material be stretched prior to wrapping, it is also important that the force of the packaging material applied to the load allow compensation for the irregular geometry of some loads.

[0006] Various techniques have been used to stretch plastic film prior to wrapping the film about the load. Friction brake systems provide a roll of film within a film carriage supported on a core shaft. The film is dispensed due to the relative rotation between the between the load and the packaging material dispenser, i.e., as rotation occurs, the film is pulled off of the roll to be wrapped around the load. Thus, film dispensing is not driven, but is passive. The film is stretched by the application of a brake directly to core of the film roll as the film is dispensed. Such friction brake systems were popular due to their simplicity. However, such systems had several drawbacks. One such drawback was the change in wrap force as the roll of film changed size. That is, as the film was dispensed, the size of the film roll necessarily decreased, and at the same time, the force being applied to the load by the stretch wrap (wrap force) increased. This necessarily required the manual intervention of the machine operator to reduce the force being applied by the brake. Since the operator would frequently forget to turn it back up again after replacing the film roll, the load wrap force would be substantially below the desired level. In addition, the roll of film was part of a stretching “tug of war,” and thus all of the imperfections of the film winding process (nicks, burrs, “feathering”) would cause the film to break prematurely.

[0007] Some of these problems were addressed by indirectly applying the brake force to the roll of film. This was accomplished by adding a roller, such as a pressure roller of a downstream roller, to the film dispenser. Instead of applying the brake to the core of the roll of film, the brake was applied to the roller. Use of the roller overcame the problem associated with the changing size of the film roll and eliminated the film “tug of war.” Such devices, however, were more expensive to produce and remained limited by the amount of variation of force applied by the brake. Thus, it was not possible to adjust the force on the film to accommodate changes in the geometry of the load, rendering the film susceptible to breakage near corners of the load.

[0008] Film driven roller stretch devices were created to address the problems associated with friction brake systems. In film driven roller stretch devices, such as those disclosed in U.S. Pat. Nos. 4,302,920 and 4,497,159, both assigned to Lantech, Inc., the packaging material is stretched between two interconnected rollers, one moving faster than the other. These rollers may be connected by friction, as disclosed in U.S. Pat. No. 4,497,159, where the two prestretch rollers are in frictional contact with one another via a cam assembly. This eliminated many of the problems associated with friction brake devices. However, in order to accommodate irregular and force sensitive loads, it was necessary to find a way to vary the wrap force of the film as it was applied to the load. This problem was addressed by the device disclosed in U.S. Pat. No. 4,302,920, where the rollers were connected via a gear/clutch assembly to allow variation in the wrap force. This device was costly to manufacture and in view of technology which emerged at approximately the same time, the powered prestretch device, it was never commercially successful.

[0009] The powered prestretch device employed two connected sets of powered rollers driven at different speeds to elongate the packaging material beyond its yield point. While the powered prestretch device overcame many of the problems associated with film driven devices, it was a complex device which was costly to produce. The powered prestretch device enabled many high volume customers to realize a cost savings on the amount of packaging material used. However, in order to accommodate irregular loads, the wrap force needed to be reduced, substantially reducing the packaging material savings. The device, therefore, was not attractive to low volume customers who dealt with “order pick” and irregular loads.

[0010] Thus, there is a need for a device which provides the benefits of powered prestretch without the costs of powered prestretch.

SUMMARY OF THE INVENTION

[0011] Accordingly, the present invention is directed to a method and apparatus for wrapping a load with packaging material which provides advantages over and obviates several problems associated with earlier methods and apparatus for prestretching packaging material.

[0012] According to one aspect of the present invention, a method of wrapping a load is provided. The method includes placing a load on a load wrapping surface, pulling a leading end of packaging material from a packaging material dispenser and attaching the leading end of packaging material to one of the load and the load wrapping surface, providing relative rotation between the load and the packaging material dispenser to dispense the packaging material, applying an initial force to prestretch the packaging material in the dispenser to a point below the packaging material yield point, and applying a secondary force to further stretch the prestretched packaging material to a point above the yield point as the packaging material is applied to the load.

[0013] According to another aspect of the present invention, a method of wrapping a load comprises placing a load on a load wrapping surface, pulling a leading end of film from a film dispenser and attaching the leading end of film to one of the load and the load wrapping surface, providing relative rotation between the load and the dispenser to dispense the film, as the film is dispensed, prestretching the film between two unpowered prestretch rollers in the dispenser to a point below the film yield point, and applying friction to one of the two prestretch rollers to stretch the film between the load and the dispenser to a point above the yield point as the film is applied to the load.

[0014] According to yet another aspect of the present invention, an apparatus for as wrapping a load is provided. The apparatus includes a dispenser for dispensing packaging material, the dispenser including a prestretch portion configured to prestretch the packaging material within the dispenser to a point below a yield point of the packaging material, a post stretch portion configured to further stretch the prestretched packaging material to a point above the yield point of the packaging material as the packaging material moves from the dispenser to the load, and means for providing relative rotation between the load and the dispenser to wrap packaging material around the load.

[0015] According to a further aspect of the present invention, an apparatus for wrapping a load comprises a dispenser for dispensing packaging material, the dispenser including a prestretch portion having first and second unpowered prestretch rollers configured to stretch the packaging material between the rollers to a point below a yield point of the packaging material, and a post stretch portion having an adjustable friction element configured apply friction to one of the first and second prestretch rollers to further stretch the prestretched packaging material to a point above the yield point of the packaging material as the packaging material moves from the dispenser to the load, and means for providing relative rotation between the load and the dispenser to wrap packaging material around the load.

[0016] Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

[0017] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

[0018] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one (several) embodiment(s) of the invention and together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a side view of an “order picked load;”

[0020]FIG. 2 is a top view of a wrapping apparatus detailing the various forces applied to packaging material during wrapping of a load;

[0021]FIG. 3 is a top view of a wrapping apparatus according to one aspect of the present invention;

[0022]FIG. 4A is an isometric view of a portion of a packaging material dispenser according to another aspect of the present invention;

[0023]FIG. 4B is an isometric view of the packaging material dispenser of FIG. 4A as viewed from a different angle;

[0024]FIG. 5A is an isometric view of a prestretch portion of a packaging material dispenser according to a further aspect of the present invention;

[0025]FIG. 5B is an isometric view of a post-stretch portion of a packaging material dispenser according to another aspect of the present invention;

[0026]FIG. 6 is a side view of a wrapping apparatus according to another aspect of the present invention; and

[0027]FIG. 7 is a front view of a wrapping apparatus according to a further aspect of the present invention.

DESCRIPTION OF THE EMBODIMENT

[0028] Reference will now be made in detail to the present embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0029] The present invention provides a method and apparatus for wrapping a load. The apparatus provided substantially reduces the problems typically associated with the wrapping of “order picked” loads with power prestretched packaging material. As embodied herein and shown in FIG. 1, “order picked loads” 124 include loads made up of differently sized components such as irregularly shaped loads, very light loads, and loads of an unstable nature.

[0030] The apparatus and method of the present invention provide the ability to use smaller, inexpensive components which are unpowered to apply packaging material to a load at a high wrap force. By controlling how and to what extent the packaging material is stretched, it is possible to achieve a high wrap force while preventing the packaging material from becoming brittle. To accomplish this, the packaging material is prestretched to a first predetermined level. The packaging material is further stretched to a second predetermined level. Such a two step stretching process offers a variety of benefits, including the ability to allow an operator to adjust the amount of friction to adapt the wrap force to each individual load, the capability to wrap the load (wrap force) at a higher force than can be achieved by prestretch alone while retaining the resilience of the film, i.e., the film remains more rubbery (less brittle) than in conventional prestretch applications, so as to accommodate any sharp corners or stress points of irregularly shaped loads.

[0031] In particular, prestretching the packaging material at a relatively low prestretch level, i.e., a level below the yield point of the packaging material, requires the application of less force to the film. The application of less force means that smaller rollers can be used, and that fewer rollers can be used, resulting in a cost savings to the customer. Additionally, by using a friction device in combination with prestretch, the friction device bears only a portion of the burden of retarding movement of the rollers and thus the friction device can also be smaller and less expensive.

[0032]FIG. 2 shows a load 24, a roll of packaging material 7, a first prestretch roller 18, a second prestretch roller 20, and packaging material 12 extending from roll 7 to load 24. The amount of force required to pull the packaging material 12 away from the roll 7 during wrapping is known as the delamination force and is represented by F_(o). The amount of force between the prestretch rollers 18, 20 that is applied to the packaging material during prestretch is represented by F₁. The amount (percentage) of elongation of the packaging material achieved during prestretch between the two prestretch rollers 18, 20 is represented by E₁. Conventionally, elongation E₁ is to a point above the yield point of the packaging material. The yield point can vary greatly dependent upon the type of packaging material used, for example, between 60% and 100% stretch. As used herein, the term “yield point” is given its conventional meaning as understood in the film industry. Generally, it is the stress point at which the film exhibits a specific deviation from proportionality in stress and strain. It also represents the point where substantial permanent deformation begins.

[0033] The total elongation of the packaging material being wrapped around the load is represented by E₂ and conventionally may be more than or less than E₁ due to the force applied between the prestretch device and the load, including recovery of the packaging material after prestretch. In conventional prestretch apparatus, E₂ has a value that is typically the same as or slightly less than E₁. As shown in FIG. 2, F₂ represents the wrap force, or the force on the packaging material as it is applied to the load. In conventional prestretch apparatus, where E₂ is approximately equal to E₁, F₂ is less than F₁, that is, the wrap force is less than the force applied to the packaging material during prestretch.

[0034] In contrast to the conventional wrapping apparatus described above, the present invention applies a force F₁ to elongate the packaging material between the prestretch rollers to a percentage of elongation E₁ that is below the yield point of the packaging material, for example, between 30-60%. The packaging material is then stretched again (post stretch) by the application of a second force (preferably friction) to a point above its yield point, increasing the wrap force F₂ on the packaging material as it is wrapped around the load to a value above that of F₁ such that F₂ is greater than F₁. During post stretch, the packaging material is stretched to a point above its yield point, for example, to 70-150% elongation.

[0035] In accordance with the present invention, an apparatus is provided for wrapping a load with packaging material. The apparatus includes a packaging material dispenser, a prestretch portion configured to prestretch the packaging material to a point below a yield point of the packaging material, a post stretch portion configured to further stretch the prestretched packaging material to a point above the yield point of the packaging material, and means for providing relative rotation between the load and the dispenser to wrap packaging material around the load.

[0036] As embodied herein and shown in FIG. 3, an apparatus 100 includes a packaging material dispenser 102. As shown in FIG. 3, packaging material dispenser 102 dispenses a sheet of packaging material 112 in a web form. Packaging material dispenser 102 includes a roll of packaging material contained within a roll carriage 108. Packaging material dispenser 102 may be mounted on a stationary mast 103 upon which roll carriage 108 can be vertically positioned to dispense packaging material from dispenser to wrap load as it rotates. A film unwind stand 116 is mounted on a base of the roll carriage 108. The stand is constructed support a roll of film 107 as the packaging material unwinds, moving from the roll of film 107 to a prestretch portion to be described below. Alternatively, roll carriage 108 of dispenser 102 may be vertically moveable on an arm 104 to dispense packaging material 112 spirally about load 124 as arm 104 rotates about load 124 (FIG. 6) or dispenser 102 may be mounted on a wrapping ring 111 to dispense packaging material 112 spirally about load 124 as dispenser 102 rotates around ring 111 and load 124 (FIG. 7). In a preferred embodiment, stretch wrap packaging material is used, however, various other packaging materials such as netting, strapping, banding, or tape can be used as well. As used herein, the terms “packaging material” and “film” are interchangeable.

[0037] According to the present invention, the apparatus includes a prestretch portion configured to prestretch film below the yield point. As embodied herein and shown in FIGS. 3-5A, a prestretch portion 110 may be provided in dispenser 102. The prestretch portion 110 is unpowered, driven by movement of the packaging material 112. Prestretch portion 110 includes a first upstream prestretch roller 118 and a second downstream prestretch roller 120. Neither of the rollers is connected to a power source, both the first and second prestretch rollers are unpowered. Prestretch portion 110 is preferably hingedly connected to dispenser 102.

[0038] First and second prestretch rollers 118, 120 are preferably mounted in prestretch portion 110. Prestretch portion 110 preferably includes a base portion 105 for supporting first and second prestretch rollers 118, 120. Prestretch portion 110 also preferably may include an upper surface 109 and an outer wall 106 between the base portion 105 and upper surface 109. Base portion 105 and upper surface 109 are hingedly connected to dispenser 102, preferably adjacent film unwind stand 116. This positioning is preferred such that, in use, upstream prestretch roller 118 rides on roll of film 107 mounted on film unwind stand 116. In use, the packaging material 112 is pulled off roll 107, passes over first upstream prestretch roller 118 to downstream prestretch roller 120 and then to load 124. The packaging material 112 pulls upstream prestretch roller 118 toward roll of film 107 such that upstream prestretch roller 118 rides on roll of film 107, preventing slippage of packaging material 112 as it is drawn from roll of film 107.

[0039] Preferably the first and second prestretch rollers 118, 120 are the same size, and have a diameter of approximately 2 inches. Although a smaller diameter roller is preferred because it is less expensive, larger rollers would work just as well. The first and second prestretch rollers are preferably spaced somewhat apart from one another The spacing of the rollers is essentially a function of managing cost. The closer the rollers are to one another, the less neckdown of the film there is and thus, more force must be applied to prestretch the film. In addition, the closer the rollers are, the greater the number of rollers that must be used to move the packaging material out beyond the dispenser and prestretch portion to the load. Thus, by spacing the prestretch rollers apart from one another, the cost of the apparatus of the present invention is decreased by reducing the number of rollers needed and by reducing the amount of force necessary to prestretch the film (which means that less robust, and therefore less expensive, rollers can be used).

[0040] Both rollers may be coated with a two-part urethane coating to enhance traction of the film over the rollers. However, it is preferred that the upstream roller (the first prestretch roller 118 located closest to film roll 107), be a uncoated steel roller to facilitate smooth unwind of film from the roll of film 107 while the downstream roller have a urethane coating. The urethane coating helps to prevent slippage of the film on the roller during prestretch.

[0041] First prestretch roller 118 is rotatably mounted on shaft 117 a and preferably comprises a cylinder. First prestretch roller 118 is preferably attached to base portion 105 of prestretch portion 110 of packaging material dispenser 102 via shaft 117 a. An upper end of first prestretch roller 118 includes a pulley 119. Pulley 119 preferably includes three grooves 119 a, 119 b, 119 c, each groove having the same diameter. Alternatively, pulley 119 may not have any grooves or may have one large groove. Pulley 119 is preferably made from a plastic material, although other suitable materials may be used.

[0042] Second prestretch roller 120 is rotatably mounted on a shaft 117 b and preferably comprises a cylinder. Second prestretch roller 120 is preferably attached to base portion 105 of prestretch portion 110 of packaging material dispenser 102 via shaft 117 b. An upper end of second prestretch roller 120 includes a pulley 122. Pulley 122 preferably includes 3 grooves 122 a, 122 b, 122 c, each groove having a different diameter. Alternatively, pulley 122 may include three stepped portions 122 a, 122 b, 122 c, each portion having a different diameter. Pulley 122 is preferably made from a plastic material although other suitable materials may be used. The three different diameter portions 122 a, 122 b, 122 c on pulley 122 correspond to three different levels of prestretch. For example, the largest diameter portion may correspond to a 30% prestretch level, the mid-size diameter portion may correspond to a 45% prestretch level, and the smallest diameter portion may correspond to a 60% prestretch level. The size of the diameter of the portion of the pulley 122 determines how fast second prestretch roller 120 will rotate relative to prestretch roller 118. The difference in speed between the first prestretch roller 118 and the second prestretch roller 120 determines the amount of prestretch applied to the film. The smaller the diameter of the pulley portion 122 a, 122 b, 122 c is in comparison to the diameter of the groove(s) of pulley 119, the faster the second prestretch roller 120 rotates with respect to first prestretch roller 118, and the greater the amount of prestretch applied to the film. Prestretch portion 110 is driven by packaging material 112 as it is pulled from roll 107, across prestretch roller 118 to be wrapped around load 124. Prestretch roller 118 is connected to prestretch roller 120, and the difference in speeds between the rollers is determined by the diameter of pulley 122, resulting in a desired elongation of the packaging material 112 as it moves between upstream prestretch roller 118 and downstream prestretch roller 120.

[0043] The first roller and the second roller are connected via pulley 119 and pulley 122. A connecting element 126 preferably includes a round urethane belt that is somewhat stretchable. The stretchability of the belt 126 allows the belt 126 to be moved by rolling between the grooves on each pulley 119, 122 in order to change or select the desired prestretch level.

[0044] According to another aspect of the present invention, the apparatus includes a post-stretch portion configured to further stretch the prestretched film above the yield point. As embodied herein and shown in FIGS. 3, 4A, 4B, and 5B, a post-stretch portion 160 may include a brake 170. Brake 170 preferably includes a spring-loaded brake shoe 174 placed against upstream prestretch roller 118. Spring-loaded brake shoe 174 may be made from any suitable friction material such as that used in automotive brake shoes. Spring loaded brake shoe 174 can be tensioned against upstream roller 118 by tightening a knob 176. Preferably, as shown in FIG. 3, knob 176 is connected to spring loaded brake shoe 174 through outer wall 106 of prestretch portion 110. The amount of pressure spring-loaded brake shoe 174 applies to upstream prestretch roller 118 corresponds to the level of post-stretch selected. Preferably, the amount of post-stretch will be sufficient to stretch the prestretched packaging material 112 to a point above its yield point. As discussed previously, the yield point of the packaging material will vary dependent upon brand and type. For example, if the packaging material is prestretched to an elongation E₁ of 60%, the post-stretch would be sufficient to cause the total elongation E₂ of the packaging material 112 to be up to 250%. In addition to controlling the brake by tensioning, it is preferable to provide a release to allow for complete release of the brake from the upstream roller 118. Brake 170 is therefore preferably connected to a lever 172 which allows for engaging and disengaging of brake 170 from upstream roller 118. Although the upstream roller 118 is braked in the preferred embodiment, this roller was selected only for ease of access. It is also possible to brake the downstream prestretch roller 120 instead.

[0045] According to the present invention, the apparatus includes means for providing relative rotation between the dispenser and the load to wrap packaging material around the load. As shown in FIG. 3, the means for providing relative rotation between the dispenser and the load may include a motor driven turntable 150 mounted on base 152 to rotate load 124 about a vertical axis 158. The turntable 150 includes a load support surface 156 upon which the load is supported as it is wrapped and preferably includes conveying means 160 for conveying load 124 into and out of the wrapping area. Alternatively, in an arrangement in which film dispenser 102 revolves around load 124, the dispenser may be carried by an arm or ring or other arrangement as shown in FIGS. 6 and 7.

[0046] In use, a preferred embodiment of the apparatus shown in FIGS. 1-5 operates as follows. A desired level of prestretch is set in the prestretch portion by connecting the first and second prestretch rollers to one another via connecting element 126. In particular, the prestretch level is chosen dependent upon which diameter portion of the pulley 122 the connector is placed around. For example, the connector 126 may be placed around pulley 119 and the middle diameter 122 b of pulley 122, selecting a medium prestretch level, for example 45%. In addition to selecting the prestretch level, the post-stretch level must also be selected. This is done by tightening or loosening knob 176 to increase or decrease the pressure of spring-loaded shoe 174 against upstream prestretch roller 118. The amount of post-stretch is dependent upon the amount of pressure applied to upstream prestretch roller 118 by shoe 174. Thus, the more pressure applied, the greater the amount of post-stretch, which translates to a greater amount of total elongation E₂ of the film. The amount of post-stretch applied should be chosen dependent on two things: 1) the hostility of the edge/corner of the load, and 2) the stability of the load. The more hostile, i.e., sharp or high stress, an edge or corner of a load is, the less post-stretch should be applied to the film, resulting in a more yielding film as it is wrapped about the load. Alternatively, a load without sharp edges can be wrapped at a much higher wrap force, indicating that a greater amount of post-stretch should be applied to the film. The less stable a load is, the less wrap force should be applied to the load to prevent dislodging or knocking over of the load, indicating that less post-stretch should be applied to the film. Conversely, a stable the load can be wrapped at a much higher wrap force, indicating that a greater amount of post-stretch should be applied to the film.

[0047] Once the desired prestretch and post-stretch levels are set, the packaging material 112 is pulled from the film roll 107 in the dispenser 102, threaded around first and second prestretch rollers 118, 120, and attached to either the load 124 or a clamp on turntable 150 adjacent the load. If desired, the brake 170 may be released by movement of lever 172 to facilitate pulling the packaging material out from the roll 107. The turntable 150 is activated, causing the packaging material to be pulled across first prestretch roller 118, causing first prestretch roller 118 to rotate. Rotation of first prestretch roller 118 is translated to second prestretch roller 120 via pulleys 119, 122 and connector 126. Second prestretch roller 120 begins to rotate at a speed faster than that of the first prestretch roller 118, the speed of the second prestretch roller a predetermined ratio of the speed of the first prestretch roller, the ratio determined by the selection of grooves 119 a, 119 b, 119 c and 122 a, 122 b, 122 c of pulleys 119 and 122, respectively, around which connector 126 is placed. As the first and second prestretch rollers rotate, the packaging material 112 is precisely elongated between the rollers 118, 120 by a percentage represented by the relative speed differential of the first and second prestretch rollers 118, 120. In conventional devices which use only prestretch rollers, at commonly used or “normal” wrap forces, the film elongation would stop when the packaging material reaches the downstream roller 120.

[0048] In the present invention however, post stretch occurs after the packaging material stretched between the first and second prestretch rollers 118, 120 reaches the downstream prestretch roller 120. Thus, the friction applied by the brake shoe 174 against upstream roller 118 applies additional pulling forces on the packaging material as the packaging material leaves the second prestretch roller 120 and moves toward the load. Thus, packaging material 112 continues to stretch as it moves from downstream prestretch roller 120 to the load 124. The fact that the film has not been stretched beyond it's yield point until it is being wrapped on the load means that the film retains some of its stretchiness or rubberiness, allowing it to conform to the shape of the load.

[0049] As the packaging material is dispensed from dispenser 102, the operator may choose to release brake 170 by pulling on lever 172 to lower the wrap force of the film as the wrapping approaches a “hot spot” such as a sharp corner or an unstable area of the load 124. Once the “hot spot” has been wrapped, the lever 172 can be released, re-engaging brake 170 to continue post-stretching packaging material 112 prior to wrapping it on the load.

[0050] Examples of the prestretch and post-stretch levels to be applied to packaging material during wrapping are set forth below. These examples are intended to be exemplary only, and are not meant to limit the invention in any way.

EXAMPLE 1

[0051] A stable load having very sharp corners is to be wrapped with a packaging material in which the yield point of the packaging material would be reached at a yield point of 100% elongation (stretch). According to the present invention, the packaging material would be prestretched to an elongation of 60%. Then, because the load is hostile (i.e., sharp corners) a relatively low amount of post-stretch would be applied, although enough post-stretch would be applied to exceed the yield point of the packaging material. Thus, the packaging material would be post-stretched until the total elongation exceeded 100%, for example, 110%.

EXAMPLE 2

[0052] A stable load having very sharp corners is to be wrapped with a packaging material in which the yield point of the packaging material would be reached at a yield point of 60% elongation (stretch). According to the present invention, the packaging material would be prestretched to an elongation of 30%-45%. Then, because the load is hostile (i.e., sharp corners) a relatively low amount of post-stretch would be applied, although enough post-stretch would be applied to exceed the yield point of the packaging material. Thus, the packaging material would be post-stretched until the total elongation exceeded 60%, for example, 70%.

EXAMPLE 3

[0053] An irregularly shaped stable load having no sharp corners is to be wrapped with a packaging material in which the yield point of the packaging material would be reached at a yield point of 100% elongation (stretch). According to the present invention, the packaging material would be prestretched to an elongation of 60%. Then, because the load is not hostile (i.e., sharp corners) a relatively high amount of post-stretch would be applied. Thus, the packaging material would be post-stretched until the total elongation exceeded 100%, for example, 150% to 250%.

EXAMPLE 4

[0054] An irregularly shaped stable load having no sharp corners is to be wrapped with a packaging material in which the yield point of the packaging material would be reached at a yield point of 60% elongation (stretch). According to the present invention, the packaging material would be prestretched to an elongation of 30%-45%. Then, because the load is not hostile (i.e., sharp corners) a relatively high amount of post-stretch would be applied. Thus, the packaging material would be post-stretched until the total elongation exceeded 60%, for example, 100% to 150%.

[0055] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. A method of wrapping a load, comprising: placing a load on a load wrapping surface; pulling a leading end of packaging material from a packaging material dispenser and attaching the leading end of packaging material to one of the load and the load wrapping surface; providing relative rotation between the load and the packaging material dispenser to dispense the packaging material; applying an initial force to prestretch the packaging material in the dispenser to a point below the packaging material yield point; and applying a secondary force to further stretch the prestretched packaging material to a point above the yield point as the packaging material is applied to the load.
 2. The method of claim 1, wherein applying the initial force includes prestretching the packaging material to an elongation of between 30% and 60%.
 3. The method of claim 1, wherein applying the initial force includes prestretching the packaging material to an elongation of 30%.
 4. The method of claim 1, wherein applying the initial force includes prestretching the packaging material to an elongation of 45%.
 5. The method of claim 1, wherein applying the initial force includes prestretching the packaging material to an elongation of 60%.
 6. The method of claim 1, wherein applying the secondary force includes stretching the film to a total elongation of between 70% and 150%.
 7. The method of claim 1, wherein applying the secondary force includes stretching the film to a total elongation of 70%.
 8. The method of claim 1, wherein applying the secondary force includes stretching the film to a total elongation of 100%.
 9. The method of claim 1, wherein applying the secondary force includes stretching the film to a total elongation of 125%.
 10. The method of claim 1, wherein applying the secondary force includes stretching the film to a total elongation of 150%.
 11. The method of claim 1, wherein providing relative rotation includes actuating a turntable.
 12. The method of claim 1, wherein providing relative rotation includes rotating the dispenser around the load.
 13. The method of claim 1, wherein applying an initial force includes stretching the packaging material between first and second rollers.
 14. The method of claim 1, wherein applying a secondary force includes applying friction.
 15. The method of claim 1, wherein applying a secondary force includes applying a brake to a prestretch roller.
 16. The method of claim 1, wherein the initial force is less than the secondary force.
 17. A method of wrapping a load, comprising: placing a load on a load wrapping surface; pulling a leading end of film from a film dispenser and attaching the leading end of film to one of the load and the load wrapping surface; providing relative rotation between the load and the dispenser to dispense the film; as the film is dispensed, prestretching the film between two unpowered prestretch rollers in the dispenser to a point below the film yield point; and applying friction to one of the two unpowered prestretch rollers to stretch the film between the load and the dispenser to a point above the yield point as the film is applied to the load.
 18. The method of claim 17, wherein providing relative rotation includes actuating a turntable.
 19. The method of claim 17, wherein providing relative rotation includes rotating the dispenser around the load.
 20. The method of claim 17, wherein applying friction includes applying a brake to one of the unpowered prestretch rollers.
 21. The method of claim 17, wherein prestretching the film includes prestretching the film to an elongation of between 30% and 60%.
 22. The method of claim 17, wherein prestretching the film includes prestretching the film to an elongation of 30%.
 23. The method of claim 17, wherein prestretching the film includes prestretching the film to an elongation of 45%.
 24. The method of claim 17, wherein prestretching the film includes prestretching the film to an elongation of 60%.
 25. The method of claim 17, wherein the film is stretched between the load and the dispenser to film to a total elongation of between 70% and 150%.
 26. The method of claim 17, wherein the film is stretched between the load and the dispenser to film to a total elongation of 70%.
 27. The method of claim 17, wherein the film is stretched between the load and the dispenser to film to a total elongation of 150%.
 28. The method of claim 17, wherein the film is stretched between the load and the dispenser to film to a total elongation of 100%.
 29. The method of claim 17, wherein the film is stretched between the load and the dispenser to film to a total elongation of 125%.
 30. The method of claim 17, further comprising releasing the friction to adjust the wrap force of the packaging material.
 31. The method of claim 1, further comprising ceasing application of the secondary force to adjust the wrap force of the packaging material.
 32. An apparatus for wrapping a load, comprising: a dispenser for dispensing packaging material, the dispenser including: a prestretch portion configured to prestretch the packaging material within the dispenser to a point below a yield point of the packaging material, and a post stretch portion configured to further stretch the prestretched packaging material to a point above the yield point of the packaging material as the packaging material moves from the dispenser to the load; and means for providing relative rotation between the load and the dispenser to wrap packaging material around the load.
 33. The apparatus of claim 32, wherein the means for providing relative rotation includes a turntable.
 34. The apparatus of claim 32, wherein the means for providing relative rotation includes a horizontal wrapping ring.
 35. The apparatus of claim 32, wherein the means for providing relative rotation includes an arm supporting the dispenser and configured to rotate around the load.
 36. The apparatus of claim 32, wherein the prestretch portion includes first and second unpowered prestretch rollers.
 37. The apparatus of claim 36, wherein the first and second prestretch rollers are connected to one another.
 38. The apparatus of claim 32, wherein the post-stretch portion includes a friction element.
 39. The apparatus of claim 32, wherein the post-stretch portion includes a brake.
 40. The apparatus of claim 39, wherein the brake is configured to apply friction to a portion of the prestretch portion.
 41. The apparatus of claim 39, wherein the post-stretch portion further includes a knob for adjusting tension of the brake.
 42. The apparatus of claim 39, wherein the brake is configured to apply friction to a prestretch roller in the prestretch portion of the dispenser.
 43. The apparatus of claim 32, wherein the prestretch portion is configured to be set at one of three levels of prestretch.
 44. The apparatus of claim 43, wherein the prestretch portion can be set for 30% prestretch, 45% prestretch, and 60% prestretch.
 45. The apparatus of claim 43, wherein the level of prestretch can be set by connecting first and second prestretch rollers.
 46. The apparatus of claim 32, wherein the post-stretch portion is configured to allow variation of the amount the prestretched packaging material is further stretched.
 47. The apparatus of claim 46, wherein the prestretched packaging material is further stretched to a total elongation of between 70% and 150%.
 48. The apparatus of claim 46, wherein the amount the prestretched packaging material is further stretched is varied by adjusting tension on a brake.
 49. The apparatus of claim 32, wherein the post-stretch portion is connected to a release lever to allow adjustment of wrap force on the load.
 50. An apparatus for wrapping a load, comprising: a dispenser for dispensing packaging material, the dispenser including: a prestretch portion having first and second unpowered rollers configured to stretch the packaging material between the rollers to a point below a yield point of the packaging material, and a post stretch portion having an adjustable friction element configured apply friction to one of the first and second rollers to further stretch the prestretched packaging material to a point above the yield point of the packaging material as the packaging material moves from the dispenser to the load; and means for providing relative rotation between the load and the dispenser to wrap packaging material around the load.
 51. The apparatus of claim 50, wherein the means for providing relative rotation includes a turntable.
 52. The apparatus of claim 50, wherein the means for providing relative rotation includes a horizontal wrapping ring.
 53. The apparatus of claim 50, wherein the means for providing relative rotation includes an arm supporting the dispenser and configured to rotate around the load.
 54. The apparatus of claim 50, wherein the first and second unpowered rollers are connected to one another.
 55. The apparatus of claim 50, wherein the adjustable friction element includes a brake.
 56. The apparatus of claim 50, wherein the prestretch portion is configured to be set at one of three levels of prestretch.
 57. The apparatus of claim 56, wherein the prestretch portion can be set for 30% prestretch, 45% prestretch, and 60% prestretch.
 58. The apparatus of claim 56, wherein the level of prestretch can be set by connecting first and second prestretch rollers.
 59. The apparatus of claim 50, wherein the prestretch portion is configured to be set at one of a high level of prestretch and a low level of prestretch.
 60. The apparatus of claim 50, wherein the post-stretch portion is configured to allow variation of the amount the prestretched packaging material is further stretched.
 61. The apparatus of claim 60, wherein the prestretched packaging material is further stretched to a total elongation of between 70% and 150%.
 62. The apparatus of claim 60, wherein the amount the prestretched packaging material is further stretched is varied by adjusting the friction element.
 63. The apparatus of claim 50, wherein the post-stretch portion is connected to a release lever to allow release of the friction element from the roller. 